Lesson 1 � Beginner

System Design kya hai?

System Design MNC interviews ki sabse important round hai. Ye tumhe poochta hai � ek large-scale system kaise design karenge✓ Chalo step by step samajhte hain.

? 15 min✓ Beginner✓ No prerequisites

System Design hota kya hai?

WHAT

System Design ek process hai jisme tum ek complex software system ko high-level mein design karte ho. Ye decide karta hai kaise components honge, kaise communicate karenge, kaise scale karenge � sab kuch architecture level pe.

WHEN

Jab bhi ek naya system banana ho ya existing system ko scale karna ho. MNC interviews mein L4+ level pe system design round mandatory hota hai � tumse poochte hain "Design Twitter" ya "Design WhatsApp".

WHERE

Amazon, Google, Microsoft, Meta � har MNC mein. Senior engineer se lekar staff engineer tak, system design interview har level pe hota hai. Ye DSA ke baad sabse weighted round hai.

Mental Model: Imagine karo tum ghar bana rahe ho. DSA hai tumhara bricks aur cement. System Design hai tumhara blueprint � kahan walls honge, kahan doors honge, kaise paani aayega, kaise bijli aayegi. Bina blueprint ke ghar nahi banta, bina system design ke scalable system nahi banta.

# System Design mein kya discuss hota hai:

1. Requirements ✓ Kya banana hai✓ Kitne users?
2. High-Level Design ✓ Kya kya components chahiye?
3. Database Design ✓ Kaunsa database✓ Schema kaisa?
4. Scaling ? 10 users se 10 million tak kaise jaoge?
5. Trade-offs ✓ Consistency vs Availability?

# Example: WhatsApp Design
Client ✓ API Gateway ✓ Chat Service ✓ Message Queue ✓ Database
 ✓ Notification Service
 ✓ Media Service (images, video)

Ye interviews mein kyun important hai?

MNC companies ko tumse ye jaanna hai ki tum sirf code nahi, balki system soch sakte ho. Ek junior engineer functions likhta hai, ek senior engineer system design karta hai.

Scale socho

10 users ke liye banana easy hai. 10 million users ke liye✓ Database kaise handle karega✓ Ek server pe sab nahi chalega � distributed system chahiye.

Trade-offs samjho

Consistency chahiye ya Availability✓ Latency kam karoon ya throughput badhaun✓ Har decision mein kuch sacrifice karna padta hai � ye samajhna system design hai.

Communication

System design interview mein tumhara communication bhi test hota hai. Clear socho, structured bolo, trade-offs explain karo � ye skills senior engineer ki hain.

System Design ke core components

Har large-scale system mein ye components hote hain. Inko samajh lo toh koi bhi system design kar loge:

# Core Components of System Design

1. Client (Frontend) ✓ Browser, Mobile App
2. API Gateway ✓ Single entry point, rate limiting
3. Load Balancer ✓ Traffic distribute karna
4. Application Servers ✓ Business logic
5. Caching ✓ Frequently accessed data cache
6. Database ✓ Data storage (SQL/NoSQL)
7. Message Queue ✓ Async communication
8. CDN ✓ Static content serving
9. Object Storage ✓ Images, videos, files

# Flow: Client ✓ CDN ✓ Load Balancer ✓ API Gateway ✓ App Server ✓ DB

Real-world example: YouTube

YouTube ek perfect system design example hai. Socho jab tum video upload karte ho:

# YouTube Video Upload Flow

1. User uploads video ✓ CDN edge server
2. Video goes to Object Storage (S3/GCS)
3. Transcoding Service ✓ Different resolutions (240p, 360p, 720p, 1080p)
4. Metadata Service ✓ Title, description, thumbnail stored in DB
5. Notification Service ✓ Subscribers ko notify

# YouTube Video Playback Flow

1. User searches ✓ Search Service (Elasticsearch)
2. Clicks video ✓ CDN se nearest server se video aata hai
3. Adaptive Bitrate ✓ Internet speed ke hisaab se quality adjust
4. Comments ✓ Separate service, separate database

# Scale: 500 hours video uploaded per minute!
# 2 billion users watch daily!

Key Insight: YouTube sab kuch separate services mein karta hai � upload, transcode, search, comments, notifications. Ye hai microservices architecture. Ek monolithic system itna scale nahi kar pata.

Interview mein kaise poochte hain?

System design interview ka format hota hai:

# Interview Format (45 minutes)

Step 1: Requirements (5 min)
 ? "Design a URL shortener like TinyURL"
 ✓ Poocho: "Kitne URLs daily?", "Read-heavy ya write-heavy?"

Step 2: High-Level Design (15 min)
 ✓ Components banao, data flow draw karo
 ? "Client ✓ API ✓ Service ✓ Database"

Step 3: Deep Dive (20 min)
 ✓ Database schema, caching strategy
 ? "Kaise scale karoge 1 billion URLs?"

Step 4: Trade-offs (5 min)
 ? "Consistency vs Availability?"
 ? "Kya choose karoge aur kyun?"

# Tips:
- Requirements clear karo pehle
- Assumptions batao (100M users, 1M writes/day)
- Trade-offs explicitly discuss karo
- Diagram banao � visual communication powerful hai

Exercise

Question: System Design mein client aur application server ke beech kaunsa component hota hai jo traffic distribute karta hai? (3 words ya kam mein answer do)

Question: Jab 1 server handle nahi kar pa raha traffic toh 10 servers lagao � ye kya kehlaata hai? (2 words)

Common mistakes

Requirements nahi poochte: Seedha design shuru mat karo. Pehle poocho � kitne users✓ Read-heavy ya write-heavy✓ Latency requirements?
Ek hi option sochte ho: Trade-offs discuss karo. "SQL vs NoSQL" mein dono ke pros/cons batao.
Scale nahi sochte: 10 users ke liye design nahi karte � 10 million ke liye socho.
Communication weak: Sirf drawing mat karo, explain bhi karo kyun ye component choose kiya.

Lesson complete?

System Design basics samajh aa gaye✓ Ab Scalability seekhte hain � vertical vs horizontal scaling kya hota hai.